Coated proppants are often used in hydraulic well fracturing to increase production rate of the well. The commercial “standard” coatings are typically a form of phenolic thermoset coating. For high temperature wells, such as those with a bottom hole temperature above about 200° F. (93° C.), precured phenolic coatings are often used due to their high load-bearing properties. The high crack closure stresses are usually above 6,000 psi, and often above 10,000 psi, so the proppant must resist such forces in order to keep the fracture cracks open and maintain fracture conductivity.
In practice, however, a variety of factors can adversely affect the performance of phenolic proppant coatings. The most important of these is premature curing of the partially cured phenolic resin in the coating due to exposure to high temperatures before the fractured strata has closed to the point that it forces particle to particle contact. Even the elevated, above-ground, temperatures found on loading docks and in shipping containers can be enough to effect curing of the coating long before it is desirable.
Recently, it has been discovered that cured, commercially acceptable, coatings can be applied to proppants using the polyurethane or polyurea reaction products of polyols and isocyanates. The details of these processes are disclosed in co-pending US patent application Ser. No. 13/099,893 (entitled “Coated and Cured Proppants”); Ser. No. 13/188,530 (entitled “Coated and Cured Proppants”); Ser. No. 13/626,055 (entitled “Coated and Cured Proppants”); Ser. No. 13/224,726 (entitled “Dual Function Proppants”); Ser. No. 13/355,969 (entitled “Manufacture of Polymer Coated Proppants”); and Ser. No. 13/837,396 (entitled “Proppant With Polyurea-Type Coating”), the disclosures of which are herein incorporated by reference. Such polyurethane and polyurea-based proppant coatings are economically and environmentally desirable for a number of reasons. Importantly, each acts like a fully cured coating for purposes of handling, shipping and introduction into a fractured field yet exhibit the inherent ability to form interparticle bonds under downhole temperatures and pressures for enhanced conductivity and to minimize proppant flowback after the well is put into production. Commercially available proppants that use such coatings are available under the designations PEARL and GARNET from Preferred Sands, Inc. of Radnor, Pa.
See also Tanguay et al. 2011/0297383 for high temperature proppant coatings made of a polycarbodiimide coating on sand and Tanguay et al. 2012/0018162 which relates to a polyamide imide proppant coating for high temperature applications.
Despite the potential benefits of interparticle bonding seen in the polyurethane and polyurea proppant coatings, there exists a continuing need in the industry for a proppant coating that exhibits a higher crush strength and resistance to crack closure stresses of 10,000 psi or more. The deformation of proppant coatings under the very high crack closure stresses that are found in high temperature/high pressure wells can be sufficient to alter pore passages and reduce the conductivity of the fractured strata.
It would also be even more desirable if proppants suitable for high temperature/high pressure strata would also exhibit some level of interparticle bond strength without the use or introduction of bond formation or polymer softening agents into the fractured strata. Such interparticle bonding would provide a further effect for retaining the coated proppants within the fractured strata despite the outflow of fluids and gases that can dislodge the proppant particulates and flush them from the strata.
Others have considered the addition of various materials into the coating on a proppant core to address one or more issues. For example, U.S. Pat. No. 4,493,875 relates to a composite proppant with a sand core and hollow, glass microspheres in an “adhesive” that bonds the microspheres to the core. A resole phenol/formaldehyde resin is used in the examples as a coating on the sand core of the proppant.
U.S. Pat. Nos. 5,422,183 and 5,597,784 teaches a proppant having a substantially cured inner resin coating, an outer resin coating, and a reinforcing agent interspersed at the inner coating/outer coating boundary, which is used in the propping of a fracture in a subterranean formation. The core of the proppant is said to be glass beads; various organic materials such as walnut shells, pecan shells, and synthetic polymers; or metallic particulates such as steel or aluminum pellets.
U.S. Pat. No. 6,406,789 describes a proppant particle made with a resin and filler material. The disclosed resins include epoxy, phenolic, a combination of a phenolic novolac polymer and a phenolic resole polymer; a cured combination of phenolic/furan resin or a furan resin to form a precured resin; or a curable furan/phenolic resin system curable in the presence of a strong acid to form a curable resin. The finely divided minerals that can be included in the resin include silica (quartz sand), alumina, mica, meta-silicate, calcium silicate, calcine, kaolin, talc, zirconia, boron and glass. Microcrystalline silica is noted as especially preferred.
U.S. Pat. No. 6,528,157 discloses a resin-coated proppant that contains fibers where at least a portion of the fibers protrude from the resin coating to interlock with fibers of other proppant particulates.
U.S. Pat. No. 7,490,667 describes a proppant having a water-soluble external coating on the proppant particle substrate and a microparticulate reinforcing and spacing agent at least partially embedded in the water-soluble external coating in a manner such that the microparticulate reinforcing agent is substantially released from the proppant particle substrate when the water-soluble coating dissolves or degrades.
U.S. Pat. No. 7,803,742 pertains to thermoset nanocomposite particulates made with carbon black, fumed silica, fumed alumina, carbon nanotubes, carbon nanofibers, cellulosic nanofibers, fly ash, polyhedral oligomeric silsesquioxanes, or mixtures thereof.
U.S. Pat. Nos. 8,006,754 and 8,006,755 describe proppants coated by a material whose electromagnetic properties change at a detectable level under a mechanical stress such as the closure stress of a fracture. A preferred proppant is described as a thermoset nanocomposite particulate substrate where the matrix material comprises a terpolymer of styrene, ethylvinylbenzene and divinylbenzene, and carbon black particulates possessing a length that is less than 0.5 microns in at least one principal axis direction incorporated as a nanofiller. Over the proppant is a coating that comprises a PZT alloy manifesting a strong piezoelectric effect or Terfenol-D manifesting giant magnetostrictive behavior to provide the ability to track in a downhole environment.
U.S. Pat. No. 8,298,667 describes the use of two ceramic layers that can contain a reinforcing agent of carbon black, fiberglass, carbon fibers, ceramic whiskers, ceramic particulates, metallic particulates, or any combination thereof.
Published US Patent Application 2012/0277130 describes a proppant made from a ceramic matrix with inorganic reinforcing fibers such as wollastonite, wollastonite concentrate, synthetic wollastonite, beta-wollastonite, enstatite, dolomite, magnesia, magnesium silicates, forsterite, steatite, olivines, silicon carbide, silicon nitride, inorganic fibers, fibers produced from slugs, commercially available inorganic crystalline fibers, alpha-alumina based fibers, alumina-silica based fibers, glass fibers.
Published US Patent Application 2013/0045901 describes the addition of nanoscale carbon black, fumed silica, fumed alumina, carbon nanotubes, carbon nanofibers, cellulosic nanofibers, natural and synthetic nanoclays, finely divided grades of fly ash, the polyhedral oligomeric silsesquioxanes, and clusters of different types of metals, metal alloys, and metal oxides for nanocomposite proppants.
Despite the advances in the field of proppant technology, there remains a need in the industry for premium proppants for medium and high pressure fields that can resist deformation under the very high crack closure stresses that are found in high temperature/high pressure wells.